This is a tutorial on how to make 8 half square triangles (HST) at a time. Whats nice with this method vs the 4 at a time is that on this, the bias is where its supposed to be and your final edges are on the straight of grain (since that has messed me up with the 4 at a time method where you sew on the straight of grain and end up with bias edges).

Math: I started with 5 squares and my HSTs were each slightly over 2 (They will ultimately finish at 1.5 in the quilt). You will lose about 1 with this method. So if you want HSTs to ultimately be 4 finished in the quilt, they would be 4.5 unfinished. You would want to start with 4.5 x 2 + 1 = 10 square. If you want 6.5 finished, 7 unfinished, you would start with 7 x 2 + 1 = 15 square.